The present invention relates to screen plates, e.g., screen cylinders and flat screen plates, for use, for example, in the pulp and paper industry for screening pulps and to methods for their manufacture.
In the formation of paper products from pulp, the pulp is typically screened such that impurities, such as sticks, shives and other undesirable pulp constituents, are removed. This screening process effectively separates the accept and reject portions of the pulp. Screening is generally performed using screen cylinders or flat plates, each of which is provided with openings therethrough for separating the accept and reject portions of the pulp.
In many such screens, whether flat or cylindrical, alternating grooves and ridges are provided in one of the surfaces of the screen. For example, see U.S. Pat. No. 4,529,520. These ridges and grooves are provided for adjusting the flow characteristics of the pulp passing through the screen. While these grooves and ridges have proven to be highly desirable, screens and cylinders with such surface configurations have not heretofore been readily and easily manufactured. For example, such screens are frequently formed by first providing a plurality of openings through a flat metal plate. A plurality of bars are then secured to the inflow side of the plate at longitudinally spaced positions therealong in parallel alignment one with the other to form the ridges and the grooves therebetween. In the case of a screen cylinder the bars are secured in a substantially axial direction. Particularly, the bars are conventionally welded along their opposite sides to the surface of the plate. The fabrication time and, hence, the cost of manufacture of screen plates of this type is quite substantial. Additionally, and importantly, the welds on the opposite sides of the bars occupy substantial space on the surface of the plate. The apertures through the plate are therefore oftentimes blocked by the welds and the number of screen apertures, holes or slots, is accordingly diminished. Welds along the bars also cause stress risers in the screen plate which is subject to heavy vibrations caused by high frequency pulses. Stress cracks and premature failures have frequently been observed in those areas. Additionally, in screen cylinders, rings are frequently secured on the outflow side of the screen cylinder to ensure the stiffness, rigidity and structural strength of the cylinder. Such rings have been secured by welding them circumferentially about the cylinder whereby the welds block a substantial number of screening openings decreasing the effective open area of the screen and consequently the screening throughput.
In conventional screen cylinders, only a limited percentage of the cylinder has screening slots, apertures and the like. This limits the flow through the screen and hence its capacity. It will be appreciated that it is not simply a matter of increasing the number of apertures through the screen plate to compensate for such reduced numbers because predetermined spacings between the apertures, holes or slots must be maintained to avoid the stapling phenomena which clogs the screen plate. Also, structural considerations have often been factors limiting screening capacity. Large bands of metal are often applied about the screen cylinder to provide structural strength and these effectively limit the effective number of openings.
Additionally, screen plates and cylinders used in the pulp and paper industry are subjected to high wear rates, even with hardened surface treatments, and are exposed to corrosive chemicals. In fact, it is not uncommon for plates of this type to completely wear out within weeks of their initial use, necessitating replacement with wholly new plates. These plates and cylinders are quite expensive.
Therefore, in accordance with the present invention, there are provided screen plates for use in the pulp and paper industry having substantially increased efficiency, flow capacity, and strength characteristics, as well as reduced manufacturing costs and fabrication times in comparison with prior methods of forming similar-type plates. Also, the present invention enables substantially reduced replacement costs of the screen plates when worn whereby the end-user does not need to bear the costs of a wholly new screen plate when replacing the worn plate.
Particularly, the present invention provides a screen formed of two separate layers. The first layer comprises a screening plate having narrow slots or small apertures and the second layer comprises a backing plate affording the screening plate the necessary structural strength for pressure screening in the pulp and paper environment.
For purposes of this description, the terms inflow and outflow surfaces designate the sides of the plates through which the flow enters and exits, respectively. The terms inflow and outflow will also refer to other features of the plates hereof, it being understood that the parts so designated lie on the respective entry and exit sides of the plate or elements thereof with reference to the direction of pulp flow. To form the screening plate, in accordance with a first preferred embodiment of the present invention, and preferably for screening low consistency pulps, the outflow face of a plate, for example, a flat plate, is provided with rows of grooves with parallel ridges or bands of material therebetween. The width of the grooves is many times the width of the resulting ridges or bands. Blind grooves are then formed in the opposite inflow face of the plate. These inflow grooves extend in a direction generally normal to the outflow grooves. These blind inflow grooves are preferably contoured and are formed in the plate to a depth such that the contoured grooves do not open through the plate, i.e., do not open through the bottoms of the outflow grooves. Slots are then formed through the bottoms of the contoured grooves and these slots open through the bottoms of the outflow grooves thereby establishing communication between the inflow and outflow sides of the plate. These slots also extend part way through the ridges or bands of material between the outflow grooves. Thus, on one side of the screening plate, for example, the inflow side, there are provided generally parallel extending contoured grooves each preferably having at least one inclined surface and which contoured grooves communicate with slots formed through the bottoms of the contoured grooves opening into the outflow grooves on the opposite side of the plate. The screening plate can be made of metal with the grooves and slots formed by machining, water jets, electric discharge mechanisms or by lasers. Additionally, materials other than metal may be utilized. For example, ceramics may be used because, as will become apparent from the ensuing description, the structural support for the screen plate is provided by a backing plate.
The backing plate in this first preferred embodiment of the present invention comprises a perforated plate having a plurality of relatively large openings in comparison with the slots through the screening plate. The size and number of the openings is, however, consistent with the requirement that the backing plate structurally supports the screening plate.
Where a flat screen plate is desired, the two flat plates are registered one with the other, with the backing plate disposed along the outflow side of the screening plate and secured one to the other, for example, by welding, soldering, riveting or adhesives. The backing plate thus engages the screening plate and forms a structural net-like supporting structure therefor.
To form a screen cylinder, the screening plate is preferably formed initially in a flat configuration as previously described. The plate is then rolled into a cylindrical configuration, the inflow contoured grooves preferably extending substantially parallel to the axis of the cylindrical plate and the outflow grooves extending circumferentially about the cylinder. (Of course, the inflow contoured grooves may extend circumferentially of the cylinder.) The edges of the screening plate are then secured, for example, by welding to one another to form the screening plate. Where an outflow screen cylinder is desired, the backing plate is rolled into a cylindrical shape having an inside diameter slightly smaller than the outside diameter of the screening plate. The backing plate is then heated to expand it, thereby enabling the cylindrical screening plate to be received within the backing plate. The backing plate is then heat-shrunk onto the screening plate. The plates are then secured one to the other, e.g., by welding, although shrink-fitting one plate onto the other may be itself sufficient to secure the plates to one another.
With this construction, it will be appreciated that the stiffness, rigidity and structural strength of the backing plate is effectively transmitted to the screening plate. The ridges on the outflow side of the screening plate bear against the inflow surface of the backing plate and afford support in those areas of the screening plate spanning the outflow grooves in the screening plate. Note that this construction also eliminates the capacity reducing support rings previously required in screen cylinders of this type, further enabling an increase in capacity through an increase in the lengths of the slots and contoured grooves to substantially the full axial length of the screen cylinder.
In this preferred form of the invention, it will be appreciated that a substantial increase in capacity or throughput is provided, as well as a capability of replacing the screening plate, once worn, with a wholly new screening plate, without replacing the backing plate. With respect to the increased capacity, it will be appreciated that the contoured grooves in a screen cylinder of this type may extend substantially the full axial length of the cylinder, i.e., terminate just short of the ends of the cylinder. The slots through the base of the contoured grooves, likewise, are substantially coextensive in length with the contoured grooves. The contoured grooves and slots therefore open into the circumferentially extending grooves on the outflow side of the screening plate, i.e., into the annular recesses between the circumferentially grooved outflow surface of the screening plate and the inner inflow surface of the backing plate. Those grooves also open into the apertures formed in the backing plate. It will be appreciated that the flow capacity of the outflow grooves of the screening plate and the apertures in the backing plate are substantially greater than the flow capacity through the contoured grooves and slots of the screening plate. By forming the slots and contoured grooves substantially the length of the cylinder, without interruption, substantially increased throughput is provided in comparison with prior screen cylinders of this type. For example, in screen cylinders previously manufactured, wide bands of metal conventionally extend in circumferential and axial directions for purposes of affording structural support to the cylinder. Such bands substantially diminished the flow through capacity of the screen cylinder. Here, however, the ridges or bands of material between the grooves on the outflow side of the screening plate are relatively small inasmuch as they do not provide substantial structural support to the screening plate. The accepts essentially flow about the bands or ridges and they effectively do not see the bands or ridges. Also, the relatively narrow, circumferential bands are partially recessed in a radial outward direction when the blind slots are cut between the bases of the contoured and outflow grooves, thereby effectively eliminating blockage in those areas of the bands and enabling flow around the bands through the slot portions directly radially in front of the bands.
As noted previously, the screening plate can be replaced when worn by breaking the welds or other securing means between the backing and screening plates. By machining, or chemical treatment if adhesives are used, the worn screening plate may be removed from the backing plate. A new screening plate may then be disposed and secured to the backing plate similarly as previously described. It will be appreciated that the screening plate may be formed of much thinner material than screen plates have previously been formed where structural considerations were necessary. Thus, when the present screening plate has worn to the point where it needs replacement (corresponding to a point where the structural integrity of prior screen plates would be brought into question or when the contoured grooves are no longer effective), the screening plate may be removed from the backing plate and replaced. The cost of this replacement with what is, in effect, a substantially wholly new screen plate, is sharply reduced, e.g., approximately halved, in comparison with the cost of a wholly new screen cylinder. Additional cost savings may be effected by using different materials for the screen plate and backing plate. For example, a lower grade metal may be used for the structural backing plate than for the screening plate.
It will also be appreciated that the arrangement of cylindrical screening and backing plates may be reversed where an inflow configuration is desired. That is, the screening plate may lie radially outwardly of the backing plate with the contoured grooves on the radially outermost inflow surface of the screening plate. Furthermore, costs savings are achieved where special treatments to the screening plates are used and which are not required for use on the backing plate. Additionally, the backing plate may be fabricated inexpensively and placed in inventory, while awaiting orders from customers for particular screen cylinders having particular sized contoured grooves. Thus, upon a customer order, the screening plate may be fabricated and fit to the inventoried backing plate. Also, increased longevity is accomplished in the present invention by minimizing the fatigue factor in the screening plate. Thus, while the screening plate is subjected to compressive forces during use, its fatigue factor is greatly reduced because it relies solely on the backing plate for structural support.
In another embodiment of the present invention, preferably for screening high consistency pulp and where an even stronger screening plate is desired, the outflow grooves of the screening plate may be formed as inflow grooves in the backing plate. In this embodiment, the backing plate may have a plurality of inflow grooves formed along its inflow surface. Blind apertures may be formed into its opposite surface and which open into the inflow grooves. The screening plate, in this latter embodiment, has a series of contoured grooves formed along its inflow surface. Along its opposite outflow surface, there is provided a plurality of blind slots in registry with the contoured grooves. Slots of reduced width are provided opening the bottom of the inflow contoured grooves into the outflow grooves and hence through the outflow surface of the screening plate. The screening plate and backing plate are then joined similarly as described above, in either flat or cylindrical configurations to form the desired flat or cylindrical screen plates. Thus, where a screen cylinder is being formed, it will be appreciated that the flow through the axially extending contoured grooves and slots enters the circumferentially extending grooves of the cylindrical backing plate and passes through its apertures.
In certain applications it is advantageous to angle one or both of the side faces of the contoured grooves of the screening plate and this can be accomplished simply by machining the side faces of the grooves to the appropriate angles. In this manner, one side of the contoured groove may, for instance, be formed perpendicularly to its base, while its opposite side is angled away from the base of the groove.
It will be also appreciated that the aggregate area of the openings through the screening plate is substantially increased in comparison with the aggregate area of the openings at like spacing provided through similar plates of the previously described prior art constructions. Thus, the present invention provides an increase in effective screen area of approximately 20 to 80% with respect to prior, similarly sized, plates. Further, the screen plates of the present invention may be manufactured at substantially reduced costs and fabrication times.
In the description of the invention in the specification and claims, the term "openings" will be used throughout. This term is used for convenience only, and is intended to encompass apertures of all shapes and sizes, including holes, slots, orifices and passageways.
In a preferred embodiment according to the present invention, there is provided a screen cylinder comprising a generally cylindrical screening medium having a plurality of openings therethrough, a generally cylindrical structural backing plate having a plurality of openings therethrough, with the screening medium and the structural backing plate lying concentrically one within the other and having respective opposed surfaces in engagement with one another at an interface therebetween whereby the backing plate structurally supports the screening medium. One of the screening medium and the backing plate has a plurality of circumferentially extending recesses formed in its opposing surface and which open at the opposing surface of the other of the screening medium and the backing plate at the interface thereof establishing communication between the respective openings of the screening medium and the backing plate.
In a further preferred embodiment according to the present invention, there is provided a screen plate for screening pulp flowing therethrough comprising a contoured screening medium having a plurality of elongated slots therethrough and extending generally parallel to one another, a structural backing plate having a plurality of openings therethrough, with the screening medium and the structural backing plate lying in registration one with the other and having respective opposed surfaces in engagement with one another at an interface therebetween whereby the backing plate structurally supports the screening medium. One of the screening medium and the backing plate has a plurality of recesses formed in its opposing surface which open at the opposing surface of the other of the screening medium and the backing plate at the interface thereof establishing communication between the respective openings of the screening medium and the backing plate, whereby pulp may flow sequentially through the contoured slots, the recesses and the openings in the backing plate.
In a further preferred embodiment according to the present invention, there is provided a method of manufacturing a screening plate for use in a screen plate for pulp, the screen plate being formed of a screening plate and a backing plate, the screening plate having first and second opposite faces, comprising the steps of (a) forming openings through the first face and into the body of the screening plate to terminate within the screening plate body and (b) forming grooves in the second face arranged to expose the openings formed in step (a) so that the openings extend entirely through the plate, and leave a plurality of ridges in the second face spaced one from the other therealong.
In a further preferred embodiment according to the present invention, there is provided a method of manufacturing a screening plate for use in a screen plate for pulp, the screen plate being formed of a screening plate and a backing plate, the screening plate having first and second opposite faces, comprising the steps of (a) forming elongated, substantially parallel, grooves in the first face, each groove being contoured and having at least two side faces and a bottom, (b) forming openings through the bottom of the grooves in the first face and into the screening plate to terminate within the screening plate short of the second face thereof (c) forming elongated grooves in the second face of the screening plate inclined relative to the longitudinal extent of the grooves formed in step (a) and to a depth to expose the openings formed in step (b) so that the openings extend entirely through the screening plate, and leave a plurality of ridges in the second face spaced one from the other therealong and extending in a direction inclined relative to the longitudinal extent of said contoured grooves.
In a further preferred embodiment according to the present invention, there is provided a method of manufacturing a screen cylinder including a discrete screening plate and a discrete backing plate comprising the steps of (a) rolling a backing plate to form a backing cylinder, (b) attaching the edges of the backing plate together to form a round cylinder, (c) rolling a screening plate to form a screening cylinder, (d) attaching the edges of the screening plate together to form a round cylinder, (e) inserting one of the screening cylinder and the backing cylinder inside the other of the cylinders and (f) shrink-fitting the cylinders into engagement one with the other.
In a further preferred embodiment according to the present invention, there is provided a method of fabricating screen cylinders for use in screening pulp and repairing a worn screen cylinder, comprising the steps of forming a screening cylinder having elongated contoured grooves along an inflow side thereof and slots through the screening cylinder for communicating pulp between inflow and outflow sides of the screening cylinder, forming a backing cylinder having a plurality of openings therethrough between inflow and outflow sides thereof, inserting one cylinder within the other cylinder, shrink-fitting the cylinders into engagement one with the other whereby the backing cylinder structurally supports the screening cylinder and enable flow of pulp through the contoured grooves and slots of the screening cylinder and the openings of the backing cylinder, subsequent to use and when worn, removing the screening cylinder from the backing cylinder, forming a second screening cylinder having elongated contoured grooves along an inflow side thereof and slots through the second screening cylinder for communicating pulp between inflow and outflow sides thereof, engaging the second screening cylinder and the backing cylinder one to the other whereby the backing cylinder structurally supports the second screening cylinder and enables flow of pulp through the contoured grooves and slots of the second screening cylinder and the openings of the backing cylinder.
Accordingly, it is a primary object of the present invention to provide novel and improved screen plates for use in the pulp and paper industry having increased efficiency and capacity for screening pulp with substantial reductions in cost and manufacturing time, as well as screen plates which can be replaced when worn at substantially reduced costs in comparison with providing a wholly new screen plate.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.